1. Field of the Invention
The present invention relates in general to metal processing techniques and, more particularly, to processes involving the fabrication or welding of advanced nickel-base superalloys.
2. Related Art
The typical nickel-base superalloy is essentially a nickel-chromium solid solution (γ phase) hardened by the addition of materials such as aluminum and/or titanium to precipitate an intermetallic compound (γ′ phase). The predominant intermetallic compound precipitated, represented by the formula Ni3 (Al, Ti), is an ordered, face centered-cubic structure with aluminum and titanium at the corners of the unit cell and nickel at the face centers. These alloys also normally contain cobalt to raise the solvus temperature of the γ′ phase, refractory metal additions for solution strengthening, and carbon, boron and zirconium to promote ductility and fabricability.
In the gas turbine engine industry in which the nickel-base superalloys are widely utilized, progressive increases in the powerplant performance requirements have led to increases in engine operating temperatures which have in turn imposed increasingly stringent demands on the turbine materials. Historically, the extent of the engine temperature increases has been limited by the physical characteristics of the hot section alloys, particularly those used in the highly stressed components such as turbine blades and vanes. Recent alloy developments, promising advances in coating technology, and the use of internal cooling techniques, however, will now allow turbine operating temperatures to be significantly increased.
Repair of gas turbine got-section components made of superalloys are often carried out by welding. These nickel-base superalloys, especially those advanced grades containing relatively high contents of the strengthening γ′ phase, are susceptible to cracking during the post-weld heat treatment that is employed to regenerate the desired microstructure-dependent properties. As a result, these superalloys are considered to be difficult to weld materials. One way to improve the weldability of this group of superalloys is to subject the materials to a pre-weld averaging treatment. This increases the material ductility which, in turn, helps to limit the buildup of residual stress produced by welding, and reduces the susceptibility to post-weld heat treatment cracking of the material. One such averaging treatment is described in U.S. Pat. No. 5,509,980 and is complex, time consuming and costly.
Accordingly, it is an object of the present invention to provide a relatively short time pre-weld preparation treatment that renders difficult or marginally weldable precipitation hardenable nickel-base superalloys readily weldable without weld-associated cracking during post-weld heat treatment.
It is a further object of the present invention to provide a relatively short time pre-weld preparation treatment that renders difficult or marginally weldable precipitation hardenable nickel-base superalloys readily weldable without the need for alloy compositional modifications and without the need for changes to otherwise conventional fusion welding procedures.